Prof. Dr. Volker Presser

Multiple principal element or high-entropy materials have recently been studied in the two-dimensional (2D) materials phase space. These promising classes of materials combine the unique behavior of solid-solution and entropy-stabilized systems with high aspect ratios and atomically thin characteristics of 2D materials. The current experimental space of these materials includes 2D transition metal oxides, carbides/carbonitrides/nitrides (MXenes), dichalcogenides, and hydrotalcites. However, high-entropy 2D materials have the potential to expand into other types, such as 2D metal-organic frameworks, 2D transition metal carbo-chalcogenides, and 2D transition metal borides (MBenes). Here, we discuss the entropy stabilization from bulk to 2D systems, the effects of disordered multi-valent elements on lattice distortion and local electronic structures and elucidate how these local changes influence the catalytic and electrochemical behavior of these 2D high-entropy materials. We also provide a perspective on 2D high-entropy materials research and its challenges and discuss the importance of this emerging field of nanomaterials in designing tunable compositions with unique electronic structures for energy, catalytic, electronic, and structural applications.

Capacitive deionization with activated carbon (AC) electrodes has been widely applied for removing charged ions from aqueous solutions. Such carbon electrodes commonly contain a minor polymer binder and a minor carbon additive content. The choice of carbon additives is rarely investigated in depth regarding the performance enhancement/deterioration they might bring. In this work, we explored the influence of various carbon types, namely, onion-like carbon, carbon black, and micro-mesoporous carbons, on the desalination capacity and rate. Based on the cycling performance of 100 cycles, we draw relationships between the physicochemical properties of different carbon types and their results on electrochemical desalination performance. The results indicate that the direct use of the activated carbon electrode without additives leads to a higher desalination capacity of approximately 10 mg/g in early cycles, though at the cost of a lower desalination rate of 6 μg/g/s. The larger AC particles limit the intraparticle ion transportation due to the increased diffusion path length. The highest desalination rate (20 μg/g/s) is enabled by the incorporation of small and less porous additives, as it shortens the ion diffusion path length due to the increased size dispersion, hence improving the overall ion transport and desalination rates.

The widespread contamination of water resources with emerging organic contaminants necessitates the development of sustainable and cost-effective water treatment technologies. Adsorption, as a widely used water remediation process, is hampered by severe performance limitations against ionic and hydrophilic organic contaminants. In addition, no facile on-site regeneration techniques are available. Electrosorption of organic compounds (EOC) is a promising alternative to not only improve adsorption performance, but also to facilitate adsorbent regeneration by green electricity. The number of studies on EOC has grown exponentially over the past decades. There are numerous examples showing that applied electric potentials can significantly enhance the adsorption affinity, capacity, and kinetics of conductive carbon adsorbents. However, whether these effects are specific to certain compound classes or more generally applicable remains unclear as well as the optimal criteria for designing EOC processes. Therefore, we critically evaluated the current state of the art of EOC in terms of active control of adsorption and desorption processes and the achievable effects for ionic and neutral organic compounds. Through a detailed consideration of compound speciation and surface chemistry of electrode materials, we derive mechanistic insights into the EOC process and discuss differences between electrosorption of inorganic and organic compounds. We provide definitions and propose insightful performance parameters to unify the rapidly growing EOC research. Potential application scenarios and future research directions are discussed. Overall, EOC is less likely to be a one-fits-all solution for removing contaminants, but adds a valuable tool especially for the hydrophilic and ionic organic contaminants that challenge conventional adsorption processes.

Ionic liquid mixtures show promise as electrolytes for supercapacitors with nanoporous electrodes. Herein, we investigate theoretically and with experiments how binary electrolytes comprising a common anion and two types of differently-sized cations affect capacitive energy storage. We find that such electrolytes can enhance the capacitance of single nanopores and nanoporous electrodes under potential differences negative relative to the potential of zero charge. For a two-electrode cell, however, they are beneficial only at low and intermediate cell voltages, while a neat ionic liquid performs better at higher voltages. We reveal subtle effects of how the distribution of pores accessible to different types of ions correlates with charge storage and suggest approaches to increase capacitance and stored energy density with ionic liquid mixtures.

Electrochemical desalination shows promise for ion-selective, energy-efficient water desalination. This work reviews performance metrics commonly used for electrochemical desalination. We provide a step-by-step guide on acquiring, processing, and calculating raw desalination data, emphasizing informative and reliable figures of merit. A typical experiment uses calibrated conductivity probes to relate measured conductivity to concentration. Using a standard electrochemical desalination cell with activated carbon electrodes, we demonstrate the calculation of desalination capacity, charge efficiency, energy consumption, and ion selectivity metrics. We address potential pitfalls in performance metric calculations, including leakage current (charge) considerations and aging of conductivity probes, which can lead to inaccurate results. The relationships between pH, temperature, and conductivity are explored, highlighting their influence on final concentrations. Finally, we provide a checklist for calculating performance metrics and planning electrochemical desalination tests to ensure accuracy and reliability. Additionally, we offer simplified spreadsheet tools to aid data processing, system design, estimations, and upscaling.

Electrochemically converting nitrate, widely distributed in industrial wastewater and contaminated water bodies, into ammonia is a promising route for resource recovery and wastewater treatment. Meanwhile, treating harmful algal blooms (HABs) is presented worldwide, are time and resource-consuming, and carries a high CO2 footprint. Rather than considering this carbon and nitrogen-rich biomass as disposable waste, consider it a vast renewable resource. Therefore, this study presents a Fe-dispersed carbon-based catalyst derived from HABs biomass, with a maximum ammonia yield rate of 16449 μg h−1 cm−2 (1.2 mmol h−1 mgcat−1) and NH3 Faradaic efficiency of 87.3%. This catalyst also possessed decent stability with continuous operation over 50 h. Experimental and theoretical calculation results reveal that the Fe-N4 site facilitates electrocatalytic nitrate reduction reaction by reducing the energy barriers of the NO3–to-NH3 pathway. Thus, this strategy of upcycling HABs biomass waste into functional catalysts offers a multipronged approach to renewable and carbon-neutral energy technologies.

Heavy metal pollution is a key environmental problem. Selectively extracting heavy metals could accomplish water purification and resource recycling simultaneously. Adsorption is a promising approach with a facile process, adaptability for the broad concentration of feed water, and high selectivity. However, the adsorption method faces challenges in synthesizing high-performance sorbents and regenerating adsorbents effectively. FeOOH is an environmentally friendly sorbent with low-cost production on a large scale. Nevertheless, the selectivity behavior and regeneration of FeOOH are seldom studied. Therefore, we investigated the selectivity of FeOOH in a mixed solution of Co2+, Ni2+, and Pb2+ and proposed to enhance the capacity of FeOOH and regenerate it by using external charges. Without charge, the FeOOH electrode shows a Pb2+ uptake capacity of 20 mg/g. After applying a voltage of −0.2/+0.8 V, the uptake capacity increases to a maximum of 42 mg/g and the desorption ratio is 70%–80%. In 35 cycles, FeOOH shows a superior selectivity towards Pb2+ compared with Co2+ and Ni2+, with a purity of 97% ± 3% in the extracts. The high selectivity is attributed to the lower activation energy for Pb2+ sorption. The capacity retentions at the 5th and the 35th cycles are ca. 80% and ca. 50%, respectively, comparable to the chemical regeneration method. With industrially exhausted granular ferric hydroxide as the electrode material, the system exhibits a Pb2+ uptake capacity of 37.4 mg/g with high selectivity. Our work demonstrates the feasibility of regenerating FeOOH by charge and provides a new approach for recycling and upcycling FeOOH sorbent.

Herein, we report the design and synthesis of a layered redox-active, antiferromagnetic metal organic semiconductor crystals with the chemical formula [Cu(H2O)2V(µ-O)(PPA)2] (where PPA is phenylphosphonate). The crystal structure of [Cu(H2O)2V(µ-O)(PPA)2] shows that the metal phosphonate layers are separated by phenyl groups of the phenyl phosphonate linker. Tauc plotting of diffuse reflectance spectra indicates that [Cu(H2O)2V(µ-O)(PPA)2] has an indirect band gap of 2.19 eV. Photoluminescence (PL) spectra indicate a complex landscape of energy states with PL peaks at 1.8 and 2.2 eV. [Cu(H2O)2V(µ-O)(PPA)2] has estimated hybrid ionic and electronic conductivity values between 0.13 and 0.6 S m−1. Temperature-dependent magnetization measurements show that [Cu(H2O)2V(µ-O)(PPA)2] exhibits short range antiferromagnetic order between Cu(II) and V(IV) ions. [Cu(H2O)2V(µ-O)(PPA)2] is also photoluminescent with photoluminescence quantum yield of 0.02%. [Cu(H2O)2V(µ-O)(PPA)2] shows high electrochemical, and thermal stability.

Due to their high energy density, Li-ion batteries have become indispensable for energy storage in many technical devices. Prussian blue and its analogs are a versatile family of materials. Apart from their direct use as an alkali-ion battery electrode, they are a promising source for templating other compounds due to the presence of carbon, nitrogen, and metallic elements in their structure, ease of synthesis, and high tunability. In this study, homogeneous iron vanadate derivatization from iron vanadium Prussian blue was successfully carried out using an energy efficient infrared furnace utilizing CO2 gas. Iron-vanadate is an inherently unstable electrode material if cycled at low potentials vs. Li/Li+. Several parameters were optimized to achieve a stable electrochemical performance of this derivative, and the effect of surfactants, such as tannic acid, sodium dodecylbenzene sulfonate, and polyvinylpyrrolidone were shown with their role in the morphology and electrochemical performance. While stabilizing the performance, we demonstrate that the type and order of addition of these surfactants are fundamental for a successful coating formation, otherwise they can hinder the formation of PBA, which has not been reported previously. Step-by-step, we illustrate how to prepare self-standing electrodes for Li-ion battery cells without using an organic solvent or a fluorine-containing binder while stabilizing the electrochemical performance. A 400 mA h g−1 capacity at the specific current of 250 mA g−1 was achieved after 150 cycles while maintaining a Coulombic efficiency of 99.2% over an extended potential range of 0.01–3.50 V vs. Li/Li+.

The controlled functionalization of surfaces is of utmost importance for many applications. Surface-initiated living anionic polymerization (SI-LAP) offers a well-adjustable, uniform functionalization without the necessity of metal catalysts for polymerization. However, this technique is rarely studied for functional monomers, such as different methacrylates. The present study investigated the SI-LAP of different methacrylate monomers on porous polystyrene microparticles. Starting with methyl methacrylate (MMA) as the model monomer, the reaction kinetics and the living character of the polymerization at the particles’ surface are discussed. The reaction conditions were transferred to more functional methacrylates, for example, 2-(trimethylsilyloxy)ethyl methacrylate (HEMA-TMS). The functionalization in the particle’s interior enables the preparation of fluorescent particles by applying post-modification protocols of the poly(hydroxyethyl methacrylate) (PHEMA) moieties with fluorescein isothiocyanate. Moreover, ferrocenylmethyl methacrylate (FMMA) polymerization leads to stimuli-responsive particles with an adjustable functional polymer content of 7 to 51%. Electrochemical studies for the latter polymer poly(ferrocenylmethyl methacrylate) (PFMMA) on the surface offered remarkable long-term stability upon addressing the redox responsiveness of the ferrocene moieties over 1000 cycles using electrochemistry. The synthesis strategy enables access to various applications, such as battery anodes, redox-flow batteries, or ion sorbents.